Municipal sewage treatment standards of China have three levels, i.e., primary treatment, secondary treatment and tertiary treatment. Primary treatment is to remove suspended solids in sewage by physical methods such as sedimentation, floatation, filtration, or to primarily clarify strong acids, strong bases and over-rich toxic substances in sewage by chemical methods such as agglomeration, oxidation, neutralization, etc., so as to provide suitable water quality conditions for secondary treatment. Secondary treatment is to further treat sewage by biochemical function on the basis of primary treatment. Tertiary treatment is to treat water according to water feed quality by a corresponding method such as coagulation sedimentation, activated carbon filtration, reverse osmosis, ion exchange and electrodialysis on the basis of secondary treatment.
Sewage biotreatment belongs to secondary treatment, aiming at removing unprecipitable suspended solids and soluble biodegradable organics, which processes can be varied, including activated sludge method, AB method, A/O method, A2/O method, SBR method, oxidation ditch method, oxidation pond method, land treatment method, etc. In the process of sewage biotreatment, temperature has comprehensive influences on microorganisms, and most microorganisms in sewage treatment have an optimum growing temperature ranging 20-30° C. Since temperature changes upon climate, temperature control is difficult in either economic or engineering aspects.
Sewage after secondary treatment still contains phosphorus, nitrogen and non-degradable organic, minerals, pathogens, and needs further purification (i.e., tertiary treatment) to remove contaminants. Although sewage tertiary treatment not only reduces pollution of wastewater discharge on environment, but also save water resources. However, both capital expenditure and operating cost of sewage tertiary treatment are relatively high, about 2-3 times the sewage secondary treatment plant of the same scale, so that its development and application are limited.
Traditional sewage treatment processes purify sewage and gather contaminants in sludge merely by biologic degradation and conversion as well as solid-liquid separation, the sludge comprising primary sludge generated in primary treatment section, residual sludge (excessive sludge) generated in secondary treatment section, and chemical sludge generated in tertiary treatment section. Since these kinds of sludge contain a large amount of organics and pathogens and are easy to corrupt and stink, they may result in secondary pollution. Hence, sludge usually must be subjected to volume reduction, amount reduction, stabilization, harmless treatment and appropriate treatment. Common sludge reduction methods comprise digestion methods (including anaerobic digestion and aerobic treatment), sludge thermal treatment methods such as wet oxidation methods, sludge concentration methods such as gravity concentration methods and air flotation concentration methods, sludge dewatering methods such as mechanical dewatering and chemical coagulation methods, sludge drying methods such as natural drying methods and oven drying methods. However, these sludge reduction methods do not completely solve the problems of sludge discharge.
Flocculation clarification apparatus (also called as: clarification apparatus, clarifier, clarification basin, flocculation sedimentation basin, coagulation basin, etc.) is to combine and complete two processes, flocculation reaction (also called as: coagulation reaction) and sedimentation, in one structure, so as to use mature floc to contact and flocculate contaminant particles, thereby separating them from water. Among common flocculation clarification apparatuses, mechanical agitation clarifier is difficult to achieve uniform mixing because of the presence of short pass water flow and the entire concentric circle movement of water body; suspension clarifier is a dreg-suspended type clarifier, which is sensitive to factors such as water inflow, water temperature, has instable treatment effects and low water production rate; hydraulic circulation clarifier uses water flow of water ejector to perform mixing and achieve dreg circulation and reflow, but traditional hydraulic circulation clarifier has poor adaptability to water quality and water temperature, is unstable in running, difficult in dreg return control, and high head loss and energy consumption.
Hence, novel sewage treatment apparatus, especially sewage treatment apparatus capable of performing sewage tertiary treatment, is still in need to meet the continuously increasing requirements of sewage treatment.